Piston pump for a vehicle brake system having a plastic piston with a wear resistant cam-striking face

ABSTRACT

A piston pump for a slip-controlled motor vehicle brake system, with a piston which, by means of a piston restoring spring, is pressed with an end face that constitutes a strike face against a circumference of a cam. The cam is driven to rotate, wherein the cam is used to drive the piston in a reciprocating stroke motion. In order to be able to inexpensively manufacture the piston as an injection molded part made of plastic, the piston is provided with a cam strike piece made of wear resistant material on the strike face which is in contact with the cam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/DE98/01355 filed on May 15, 1998.

BACKGROUND OF THE INVENTION

The invention relates to a piston pump for a brake of a vehicle.

A piston pump of this kind for a hydraulic, slip-controlled vehiclebrake system has been disclosed by DE 41 07 979 A1. The known pistonpump has a pump housing with a cylinder bore in which a piston iscontained so that the piston can move axially. A cam element that can bedriven to rotate is provided for driving the piston into a reciprocatingstroke motion in the axial direction; this cam element is disposedagainst an end face of the piston and the piston includes an end facethat rests against a circumference which constitutes a strike surface.The piston of the known piston pump is a costly, multiple-step workpiece that is manufactured in a cutting fashion by means of turning andboring. A circumference surface of the piston must be ground since itconstitutes a bearing surface of the piston.

ADVANTAGES OF THE INVENTION

The piston pump according to the invention, has a piston made ofplastic, which is manufactured, for example, by means of injectionmolding. In order for the end face of the piston, which constitutes thestrike surface and which rests against the cam element, to be able todurably withstand the stresses occurring between the cam element and thestrike surface predominantly due to friction, the piston has a camstrike piece made of wear resistant material on its strike surface. Thecam strike piece is for example disk shaped, it is preferably insertedinto an injection molding tool before the piston is injection molded,and the plastic of the piston is injection molded around the cam strikepiece, wherein the strike face provided for contacting the cam elementis not covered with plastic. The cam strike piece can, for example, becomprised of a metal such as hardened steel, or of ceramic, for examplesintered ceramic.

Firstly, the piston pump according to the invention has an advantagethat its piston can be simply, rapidly, and inexpensively manufacturedin one work cycle by means of injection molding. A finishing, forexample by means of grinding the piston circumference or shaping a valveseat disposed in the piston, can be eliminated. Fluid conduits possiblyrequired in the piston for the fluid inlet into the piston pump or forthe fluid outlet from the piston pump are produced with forming coresinserted into the injection molding tool so that finishing is notrequired for the inlet or outlet cores.

Other advantages of the invention are positive sliding properties of thepiston in the pump housing due to the fact that the piston is made ofplastic. In order to improve the sliding properties and to reduce wear,the plastic of the piston can have TEFLON components added to it. If avalve that controls the fluid inlet or the fluid outlet is attached tothe piston, whose valve seat is embodied on the piston, the elasticitybehavior of the piston made of plastic improves the closing behavior andtightness of the valve, particularly with increasing pressure.

The piston can also be used to produce a seal between the piston and thepump housing. This has the advantage that a sealing ring can be omitted.

Another advantage of the invention is that without additional expense, astepped piston can be produced in which one end of the piston has agreater diameter than the other end of the piston.

Advantageous embodiments and improvements of the invention disclosed area subject of the invention disclosed hereinafter.

With an embodiment of the invention, the cam strike piece has positiveengagement elements on an outside which produce a positive engagementwith the piston in order to improve the securing of the cam strike piecein the piston. Positive engagement elements of this kind can becontinuous or interrupted ribs, grooves, or the like.

The piston pump according to the invention is provided in particular asa pump in a brake system of a vehicle and is used to control thepressure in wheel brake cylinders. The abbreviations ABS, ASR, FDR, orEHB are used for such brake systems, depending on the type of brakesystem. In the brake system, the pump serves for instance to returnbrake fluid from a wheel brake cylinder or a plurality of wheel brakecylinders to a master cylinder (ABS) and/or to supply brake fluid out ofa storage tank into a wheel brake cylinder or a plurality of wheel brakecylinders (ASR, FDR, or EHB). The pump is required, for example, in abrake system with wheel slip control (ABS or ASR) and/or a brake systemserving as a steering aid (FDR) and/or an electrohydraulic brake system(EHB). With wheel slip control (ABS or ASR), for example, a locking ofthe wheels of the vehicle during a braking event when there is strongpressure on the brake pedal (ABS) and/or a spinning of the driven wheelsof the vehicle when there is strong pressure on the gas pedal (ASR) canbe prevented. In a brake system that serves as a steering aid (FDR), abrake pressure is built up in one or more wheel brake cylindersindependently of an actuation of the brake pedal or gas pedal, forinstance to prevent the vehicle from breaking out of the track desiredby the driver. The pump can also be used in an electrohydraulic brakesystem (EHB) in which the pump supplies the brake fluid into the wheelbrake cylinder or cylinders if an electrical brake pedal sensor detectsan actuation of the brake pedal or in which the pump is used to fill areservoir of the brake system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below in conjunction witha preferably selected exemplary embodiment shown in the drawing.

The sole FIGURE shows an axial section through a piston pump accordingto the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

The piston pump 10 according to the invention, which is shown in thedrawing, is inserted into a cylinder bore 12 in a pump housing 14, whichis constituted by a hydraulic block of a vehicle brake system that isotherwise not shown. Other hydraulic components that are not shown, suchas solenoid valves and the like, are inserted into the hydraulic block,of which only a fraction that encompasses the piston pump 10 is shown inthe drawing for the sake of clarity, and are hydraulically connected toone another and to the piston pump 10. The hydraulic block, togetherwith the hydraulic components inserted into it, including the pistonpump 10, is part of a slip-control mechanism of a motor vehicle brakesystem. The hydraulic block is connected in an intrinsically knownmanner to a master cylinder, not shown, and wheel brake cylinders, notshown, are hydraulically connected to the hydraulic block 10.

The piston pump 10 has a sleeve 16 with a sleeve bottom 18 that is anintegral part of the sleeve and is of a greater diameter and ispressed-fitted into the pump bore 12. By means of a crimp 20, acylindrical stopper 22 is affixed to the sleeve bottom 18 and seals anend of the pump bore 12 in a pressure-tight manner by means of acaulking 24 of the pump housing 14, and fixes the sleeve 16 in the pumphousing 14. As an outlet valve 28, a check valve is inserted into ablind hole 26 of the stopper 22 and has a valve ball 30 as a valveclosing body, which is pressed by a helical compression spring as avalve closing spring 32 against a conical valve seat 34 which isembodied at a mouth of an axial through hole 34 in the sleeve bottom 18.An outlet takes place by means of an outlet conduit 38 that isconstituted by a radial groove in the stopper 22 and communicates withan outlet bore 40 in the pump housing 14.

A piston 42 of the piston pump 10 according to the invention is anessentially cylindrical injection molded part made of plastic, whichprotrudes a short way out from the sleeve 16. The piston 42 is comprisedof a carbon fiber reinforced plastic which has TEFLON components addedto it in order to improve its sliding properties. On its end protrudingfrom the sleeve 16, the piston 42 is axially guided directly in the pumpbore 12 of the pump housing 14. A seal between the pump housing 14 andthe piston 42 is produced by means of an O-ring 44, which is insertedinto an annular step-shaped shoulder of the pump bore 12 and is heldthere by an end face of the sleeve 16. In order to guide an end thepiston disposed in the sleeve 16, the piston 42 has a collar 46 that isof one piece with the piston, with which the piston 42 is axially guidedin the sleeve 16. A sliding ring or the like is not required. A sealbetween the sleeve 16 and the piston 42 at the end of the piston 42disposed in the sleeve 16 is produced by means of a rubber elasticelastomer sealing ring 48, which is slid onto the piston 42 from an endof the collar 46 oriented toward a displacement chamber 50 of the pistonpump 10. The displacement chamber 50 is encompassed by the sleeve 16; itis defined on its end faces on the one hand by the sleeve bottom 18 andon the other hand by the piston 42 with the elastomer sealing ring 48. Avolume of the displacement chamber 50 increases and decreases with anaxial reciprocating stroke motion of the piston 42 in the sleeve 16, bymeans of which fluid is aspirated by the piston pump 10 in anintrinsically known manner and is discharged or/supplied thereto.

A cam 52 that can be driven to rotate by an electric motor is disposedagainst the end face of the piston 42 protruding from the sleeve 16 andthe piston 42 rests with its end face 54 against the circumferencesurface of this cam. The end face of the piston resting against thecircumference of the cam 52 constitutes a strike face 54 of the piston42. Since during operation of the piston pump 10, a friction occursbetween the circumference of the cam 52 and the strike face 54 of thepiston 42 due to the rotating drive of the cam 52, the piston 42 has acam strike piece 56 made of wear resistant material, which constitutesthe strike face 54. The cam strike piece 56 is disk-shaped and in theexemplary embodiment depicted, is made of hardened steel. In themanufacturing of the piston, the cam strike piece 56 is inserted into aninjection molding tool, not shown, and the plastic that constitutes thepiston 42 is injection molded around the strike piece, wherein thestrike face 54 remains free of plastic and the cam strike piece 56 isotherwise completely embedded in the plastic of the piston 42. On itscircumference the cam strike piece 56 has a continuous groove 58 that issemicircular in cross section and produces a positive engagement betweenthe plastic of the piston 42 and the cam strike piece 56, and thusimproves a securing of the cam strike piece 56 in the piston 42.

A piston restoring spring 60 in the form of a helical compressionspring, which is inserted into the displacement chamber 50, acts on thepiston 42, and is supported against the sleeve bottom 18, presses thepiston 42 against the cam 52, i.e. the piston restoring spring 60 holdsthe piston 42 so that its strike face 54 contacts the circumference ofthe cam 52.

For the fluid inlet, the piston pump 10 according to the invention hasan inlet bore 62 that is let into the pump housing 14 radial to the pumpbore 12 and feeds fluid into the pump bore 12. The inlet bore 62 in thepump housing 14 communicates with an internal chamber of the sleeve 16by means of inlet bores 64 in the circumference of the sleeve 16. Thepiston 42 is provided with criss-crossing lateral holes 66 approximatelyin the center of its length, which cross an axial blind hole 68 close toits bottom. The blind hole 68 lets fluid out by forming a conical valveseat 70 of an inlet valve 72 of the piston pump 10 on the end face ofthe piston 42 oriented toward the displacement chamber 50. A flat, wide,circumferential groove 74, which is disposed in the piston 42 and hasthe lateral holes 66 let into its base, assures that the lateral holes66 communicate with the inlet bores 62, 64 in every stroke position ofthe piston 42. With the collar 46 and the groove 74, the piston 42 has anumber of steps that can be produced with no trouble by injectionmolding the piston 42. The lateral holes 66, which constitute a part ofthe fluid inlet of the piston pump 10, and the blind hole 68 in thepiston 42 are formed in the piston production with forming coresinserted into the injection molding tool so that a separate work cycleis not required to make the lateral holes 66 and the blind hole 68 inthe piston 42.

The inlet valve 72 is affixed to the end face of the piston 42 orientedtoward the displacement chamber 50. The inlet valve 72 is embodied as aspring-loaded check valve. It has a valve ball 76 as the valve closingbody which is pressed against the valve seat 70 at the mouth of theblind hole 68 of the piston 42 by a helical compression spring thatconstitutes a valve closing spring 78. The valve ball 76 and the valveclosing spring 78 are contained in a cup-shaped valve cage 80, which ismanufactured out of sheet metal as a deep-drawn part. The valve closingspring 78 is supported against a bottom of the valve cage 80. The valvecage 80 is provided with fluid openings 82 in the circumference andbottom. On an open end, an annular step 84 is formed onto the valve cage80, with which the valve cage 80 is slid onto the end face of the piston42 oriented toward the displacement chamber 50. A free edge of the valvecage 80 is formed into a washer-shaped spring plate 86 that protrudesradially outward. The piston restoring spring 60 engages the piston 42by means of the spring plate 86. At the same time, the spring plate 86secures the elastomer sealing ring 48, which is inserted between it andthe collar 46 of the piston 42, axially to the piston 42. An axialspacing between the collar 46 and the spring plate 86 is greater than awidth of the elastomer sealing ring 48 so that the elastomer sealingring 48 is not axially compressed. The piston restoring spring 60 isembodied as considerably stronger than the valve closing spring 78 ofthe inlet valve 72 so that with all loads that occur during operation ofthe piston pump 10, the piston restoring spring 60 holds the piston sothat the strike face 54 contacts the circumference of the cam 52 andholds the valve cage 18 against the piston 42 counter to the force ofthe valve closing spring 78.

Since the elastomer sealing ring 48 is slid onto the piston 42 beforethe valve cage 80 is attached to the piston 42, the assembly of theseparts is very simple, in particular, the elastomer sealing ring 48 doesnot have to be deformed during assembly or only needs to be deformed toan insignificant degree.

Since the piston 42 is comprised of a soft material, namely of plastic,a piston gap between the sleeve 16 and the piston 42 or its collar 46can be embodied as very narrow. The piston 42 can even be inserted intothe sleeve 16 with a slight degree of initial stress. The piston gap ofthe piston pump 10 proposed here can be embodied as significantlynarrower than, for example, in the case of a piston made of metal. Dueto the narrow piston gap between the piston 42 and the sleeve 16, thereis no danger that as a result of high pressure in the displacementchamber 50, the elastomer sealing ring 48 will be pinched into thepiston gap between the piston 42 and the sleeve 16.

Should the need arise, it is also possible to eliminate the sleeve 16and to guide the piston 42 so that the piston can axially move directlyin the pump housing 14 without a sleeve (not shown). As a result, thenumber of required components can be reduced. This is aided by themanufacture of the piston 42 out of plastic, because even with theelimination of the sleeve 16, this assures a good, low-friction,long-lasting guidance of the piston 42 directly in the pump housing 14.

The foregoing relates to a preferred exemplary embodiment of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

In the exemplary embodiment shown, the piston 42 is embodied as being ofthe same diameter on both its ends. It is also possible to embody thepiston as being of different diameters, i.e. to embody the piston asstepped in order, for example, to improve an aspiration behavior of thepiston pump 10 (not shown). Since it is easily possible to give thepiston 42 a desired, for example stepped, form with different pistondiameters over the length of the piston, embodying the piston as astepped piston does not increase the cost.

What is claimed is:
 1. A piston pump for a vehicle brake system,comprising a pump housing a cylinder bore (12) in said housing, a sleeve(18) press-fitted into said cylinder bore, a piston guided in said boreso that the piston moves axially, said piston including an end face, aflat surface cam strike face on said end face, a rotatable cam elementdisposed to abutted said flat surface cam strike face on said end faceof the piston for driving said piston in a reciprocating motion in saidpiston guide, the piston (42) comprises a plastic, and the cam strikeface (56) is made of wear resistant material upon which the piston (42)rests as the cam element (52) drives the piston in a reciprocatingmotion.
 2. The piston pump according to claim 1, in which the cam strikepiece (56) is disk-shaped.
 3. The piston pump according to claim 2, inwhich the cam strike piece (56) has a positive engagement element (58)engaged with said end face of the piston (42).
 4. The piston pumpaccording to claim 2, in which the cam strike piece (56) is metal. 5.The piston pump according to claim 2, in which the cam strike piece (56)is ceramic.
 6. The piston pump according to claim 2, in which the piston(42) is an injection molded part, and that the end of the piston (42) isplastic which is injection molded around the cam strike piece (56). 7.The piston pump according to claim 2, in which the piston (42) ispolytetrafluorethylene.
 8. The piston pump according to claim 1, inwhich the cam strike piece (56) has a positive engagement element (58)engaged with said end face of the piston (42).
 9. The piston pumpaccording to claim 8, in which the cam strike piece (56) is metal. 10.The piston pump according to claim 8, in which the cam strike piece (56)is ceramic.
 11. The piston pump according to claim 8, in which thepiston (42) is an injection molded part, and that the end of the piston(42) is plastic which is injection molded around the cam strike piece(56).
 12. The piston pump according to claim 8, in which the piston (42)is polytetrafluorethylene.
 13. The piston pump according to claim 1, inwhich the cam strike piece (56) is metal.
 14. The piston pump accordingto claim 13, in which the piston (42) is an injection molded part, andthat the end of the piston (42) is plastic which is injection moldedaround the cam strike piece (56).
 15. The piston pump according to claim13, in which the piston (42) is polytetrafluorethylene.
 16. The pistonpump according to claim 1, in which the cam strike piece (56) isceramic.
 17. The piston pump according to claim 16, in which the piston(42) is polytetrafluorethylene.
 18. The piston pump according to claim1, in which the piston (42) is an injection molded part, and that theend of the piston (42) is plastic which is injection molded around thecam strike piece (56).
 19. The piston pump according to claim 18, inwhich the piston (42) is polytetrafluorethylene.
 20. The piston pumpaccording to claim 1, in which the piston (42) ispolytetrafluorethylene.